As filtering media for air filters, there have been used those produced by blending glass fibers with binders and processed into a paper. However, these filtering media suffer from some problems, for example, the existence of fine fibers adhering thereto and self-dusting at the bending process. Further, when it contacts with specific chemicals such as hydrofluoric acid, these filtering media are deteriorated and undergo dusting.
PTFE is a clean material which is highly resistant to chemicals. Due to these characteristics, porous PTFE membranes have been employed in filtering media as high-performance for air filters to be used in clean rooms in the semiconductor industry. A porous PTFE membrane disclosed in JP-A-5-202217 is one example thereof (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). It has been a practice to laminate a porous PTFE membrane onto an air-permeable supporting member for reinforcement to give a filtering medium. The thus obtained filtering medium is pleated so as to give a continuous W-shaped structure (hereinafter referred to as "pleating") and framed, thus giving an air filter.
Pleating methods commonly employed in the art involve the reciprocating method and the rotary method. The rotary method is advantageous from the viewpoint of productivity, while the reciprocating method is advantageous in giving a favorable pleated appearance. When a filtering medium composed of a porous PTFE membrane and an air-permeable supporting member for reinforcement is pleated, there arises a problem that the filtering medium is damaged and thus pinholes are formed. As a result, the filter thus obtained suffers from leakage.